001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021import static com.google.common.base.Preconditions.checkState;
022import static com.google.common.base.Predicates.instanceOf;
023import static com.google.common.collect.CollectPreconditions.checkRemove;
024import static com.google.common.collect.NullnessCasts.uncheckedCastNullableTToT;
025import static java.util.Objects.requireNonNull;
026
027import com.google.common.annotations.GwtCompatible;
028import com.google.common.annotations.GwtIncompatible;
029import com.google.common.base.Function;
030import com.google.common.base.Objects;
031import com.google.common.base.Optional;
032import com.google.common.base.Preconditions;
033import com.google.common.base.Predicate;
034import com.google.common.primitives.Ints;
035import com.google.errorprone.annotations.CanIgnoreReturnValue;
036import java.util.ArrayDeque;
037import java.util.Arrays;
038import java.util.Collection;
039import java.util.Collections;
040import java.util.Comparator;
041import java.util.Deque;
042import java.util.Enumeration;
043import java.util.Iterator;
044import java.util.List;
045import java.util.NoSuchElementException;
046import java.util.PriorityQueue;
047import java.util.Queue;
048import javax.annotation.CheckForNull;
049import org.checkerframework.checker.nullness.qual.NonNull;
050import org.checkerframework.checker.nullness.qual.Nullable;
051
052/**
053 * This class contains static utility methods that operate on or return objects of type {@link
054 * Iterator}. Except as noted, each method has a corresponding {@link Iterable}-based method in the
055 * {@link Iterables} class.
056 *
057 * <p><i>Performance notes:</i> Unless otherwise noted, all of the iterators produced in this class
058 * are <i>lazy</i>, which means that they only advance the backing iteration when absolutely
059 * necessary.
060 *
061 * <p>See the Guava User Guide section on <a href=
062 * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables">{@code
063 * Iterators}</a>.
064 *
065 * @author Kevin Bourrillion
066 * @author Jared Levy
067 * @since 2.0
068 */
069@GwtCompatible(emulated = true)
070@ElementTypesAreNonnullByDefault
071public final class Iterators {
072  private Iterators() {}
073
074  /**
075   * Returns the empty iterator.
076   *
077   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
078   */
079  static <T extends @Nullable Object> UnmodifiableIterator<T> emptyIterator() {
080    return emptyListIterator();
081  }
082
083  /**
084   * Returns the empty iterator.
085   *
086   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
087   */
088  // Casting to any type is safe since there are no actual elements.
089  @SuppressWarnings("unchecked")
090  static <T extends @Nullable Object> UnmodifiableListIterator<T> emptyListIterator() {
091    return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
092  }
093
094  /**
095   * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
096   * referenced by emptyModifiableIterator().
097   */
098  private enum EmptyModifiableIterator implements Iterator<Object> {
099    INSTANCE;
100
101    @Override
102    public boolean hasNext() {
103      return false;
104    }
105
106    @Override
107    public Object next() {
108      throw new NoSuchElementException();
109    }
110
111    @Override
112    public void remove() {
113      checkRemove(false);
114    }
115  }
116
117  /**
118   * Returns the empty {@code Iterator} that throws {@link IllegalStateException} instead of {@link
119   * UnsupportedOperationException} on a call to {@link Iterator#remove()}.
120   */
121  // Casting to any type is safe since there are no actual elements.
122  @SuppressWarnings("unchecked")
123  static <T extends @Nullable Object> Iterator<T> emptyModifiableIterator() {
124    return (Iterator<T>) EmptyModifiableIterator.INSTANCE;
125  }
126
127  /** Returns an unmodifiable view of {@code iterator}. */
128  public static <T extends @Nullable Object> UnmodifiableIterator<T> unmodifiableIterator(
129      Iterator<? extends T> iterator) {
130    checkNotNull(iterator);
131    if (iterator instanceof UnmodifiableIterator) {
132      @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
133      UnmodifiableIterator<T> result = (UnmodifiableIterator<T>) iterator;
134      return result;
135    }
136    return new UnmodifiableIterator<T>() {
137      @Override
138      public boolean hasNext() {
139        return iterator.hasNext();
140      }
141
142      @Override
143      @ParametricNullness
144      public T next() {
145        return iterator.next();
146      }
147    };
148  }
149
150  /**
151   * Simply returns its argument.
152   *
153   * @deprecated no need to use this
154   * @since 10.0
155   */
156  @Deprecated
157  public static <T extends @Nullable Object> UnmodifiableIterator<T> unmodifiableIterator(
158      UnmodifiableIterator<T> iterator) {
159    return checkNotNull(iterator);
160  }
161
162  /**
163   * Returns the number of elements remaining in {@code iterator}. The iterator will be left
164   * exhausted: its {@code hasNext()} method will return {@code false}.
165   */
166  public static int size(Iterator<?> iterator) {
167    long count = 0L;
168    while (iterator.hasNext()) {
169      iterator.next();
170      count++;
171    }
172    return Ints.saturatedCast(count);
173  }
174
175  /** Returns {@code true} if {@code iterator} contains {@code element}. */
176  public static boolean contains(Iterator<?> iterator, @CheckForNull Object element) {
177    if (element == null) {
178      while (iterator.hasNext()) {
179        if (iterator.next() == null) {
180          return true;
181        }
182      }
183    } else {
184      while (iterator.hasNext()) {
185        if (element.equals(iterator.next())) {
186          return true;
187        }
188      }
189    }
190    return false;
191  }
192
193  /**
194   * Traverses an iterator and removes every element that belongs to the provided collection. The
195   * iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
196   *
197   * @param removeFrom the iterator to (potentially) remove elements from
198   * @param elementsToRemove the elements to remove
199   * @return {@code true} if any element was removed from {@code iterator}
200   */
201  @CanIgnoreReturnValue
202  public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
203    checkNotNull(elementsToRemove);
204    boolean result = false;
205    while (removeFrom.hasNext()) {
206      if (elementsToRemove.contains(removeFrom.next())) {
207        removeFrom.remove();
208        result = true;
209      }
210    }
211    return result;
212  }
213
214  /**
215   * Removes every element that satisfies the provided predicate from the iterator. The iterator
216   * will be left exhausted: its {@code hasNext()} method will return {@code false}.
217   *
218   * @param removeFrom the iterator to (potentially) remove elements from
219   * @param predicate a predicate that determines whether an element should be removed
220   * @return {@code true} if any elements were removed from the iterator
221   * @since 2.0
222   */
223  @CanIgnoreReturnValue
224  public static <T extends @Nullable Object> boolean removeIf(
225      Iterator<T> removeFrom, Predicate<? super T> predicate) {
226    checkNotNull(predicate);
227    boolean modified = false;
228    while (removeFrom.hasNext()) {
229      if (predicate.apply(removeFrom.next())) {
230        removeFrom.remove();
231        modified = true;
232      }
233    }
234    return modified;
235  }
236
237  /**
238   * Traverses an iterator and removes every element that does not belong to the provided
239   * collection. The iterator will be left exhausted: its {@code hasNext()} method will return
240   * {@code false}.
241   *
242   * @param removeFrom the iterator to (potentially) remove elements from
243   * @param elementsToRetain the elements to retain
244   * @return {@code true} if any element was removed from {@code iterator}
245   */
246  @CanIgnoreReturnValue
247  public static boolean retainAll(Iterator<?> removeFrom, Collection<?> elementsToRetain) {
248    checkNotNull(elementsToRetain);
249    boolean result = false;
250    while (removeFrom.hasNext()) {
251      if (!elementsToRetain.contains(removeFrom.next())) {
252        removeFrom.remove();
253        result = true;
254      }
255    }
256    return result;
257  }
258
259  /**
260   * Determines whether two iterators contain equal elements in the same order. More specifically,
261   * this method returns {@code true} if {@code iterator1} and {@code iterator2} contain the same
262   * number of elements and every element of {@code iterator1} is equal to the corresponding element
263   * of {@code iterator2}.
264   *
265   * <p>Note that this will modify the supplied iterators, since they will have been advanced some
266   * number of elements forward.
267   */
268  public static boolean elementsEqual(Iterator<?> iterator1, Iterator<?> iterator2) {
269    while (iterator1.hasNext()) {
270      if (!iterator2.hasNext()) {
271        return false;
272      }
273      Object o1 = iterator1.next();
274      Object o2 = iterator2.next();
275      if (!Objects.equal(o1, o2)) {
276        return false;
277      }
278    }
279    return !iterator2.hasNext();
280  }
281
282  /**
283   * Returns a string representation of {@code iterator}, with the format {@code [e1, e2, ..., en]}.
284   * The iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
285   */
286  public static String toString(Iterator<?> iterator) {
287    StringBuilder sb = new StringBuilder().append('[');
288    boolean first = true;
289    while (iterator.hasNext()) {
290      if (!first) {
291        sb.append(", ");
292      }
293      first = false;
294      sb.append(iterator.next());
295    }
296    return sb.append(']').toString();
297  }
298
299  /**
300   * Returns the single element contained in {@code iterator}.
301   *
302   * @throws NoSuchElementException if the iterator is empty
303   * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
304   *     iterator is unspecified.
305   */
306  @ParametricNullness
307  public static <T extends @Nullable Object> T getOnlyElement(Iterator<T> iterator) {
308    T first = iterator.next();
309    if (!iterator.hasNext()) {
310      return first;
311    }
312
313    StringBuilder sb = new StringBuilder().append("expected one element but was: <").append(first);
314    for (int i = 0; i < 4 && iterator.hasNext(); i++) {
315      sb.append(", ").append(iterator.next());
316    }
317    if (iterator.hasNext()) {
318      sb.append(", ...");
319    }
320    sb.append('>');
321
322    throw new IllegalArgumentException(sb.toString());
323  }
324
325  /**
326   * Returns the single element contained in {@code iterator}, or {@code defaultValue} if the
327   * iterator is empty.
328   *
329   * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
330   *     iterator is unspecified.
331   */
332  @ParametricNullness
333  public static <T extends @Nullable Object> T getOnlyElement(
334      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
335    return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
336  }
337
338  /**
339   * Copies an iterator's elements into an array. The iterator will be left exhausted: its {@code
340   * hasNext()} method will return {@code false}.
341   *
342   * @param iterator the iterator to copy
343   * @param type the type of the elements
344   * @return a newly-allocated array into which all the elements of the iterator have been copied
345   */
346  @GwtIncompatible // Array.newInstance(Class, int)
347  public static <T extends @Nullable Object> T[] toArray(
348      Iterator<? extends T> iterator, Class<@NonNull T> type) {
349    List<T> list = Lists.newArrayList(iterator);
350    return Iterables.<T>toArray(list, type);
351  }
352
353  /**
354   * Adds all elements in {@code iterator} to {@code collection}. The iterator will be left
355   * exhausted: its {@code hasNext()} method will return {@code false}.
356   *
357   * @return {@code true} if {@code collection} was modified as a result of this operation
358   */
359  @CanIgnoreReturnValue
360  public static <T extends @Nullable Object> boolean addAll(
361      Collection<T> addTo, Iterator<? extends T> iterator) {
362    checkNotNull(addTo);
363    checkNotNull(iterator);
364    boolean wasModified = false;
365    while (iterator.hasNext()) {
366      wasModified |= addTo.add(iterator.next());
367    }
368    return wasModified;
369  }
370
371  /**
372   * Returns the number of elements in the specified iterator that equal the specified object. The
373   * iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
374   *
375   * @see Collections#frequency
376   */
377  public static int frequency(Iterator<?> iterator, @CheckForNull Object element) {
378    int count = 0;
379    while (contains(iterator, element)) {
380      // Since it lives in the same class, we know contains gets to the element and then stops,
381      // though that isn't currently publicly documented.
382      count++;
383    }
384    return count;
385  }
386
387  /**
388   * Returns an iterator that cycles indefinitely over the elements of {@code iterable}.
389   *
390   * <p>The returned iterator supports {@code remove()} if the provided iterator does. After {@code
391   * remove()} is called, subsequent cycles omit the removed element, which is no longer in {@code
392   * iterable}. The iterator's {@code hasNext()} method returns {@code true} until {@code iterable}
393   * is empty.
394   *
395   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
396   * should use an explicit {@code break} or be certain that you will eventually remove all the
397   * elements.
398   */
399  public static <T extends @Nullable Object> Iterator<T> cycle(Iterable<T> iterable) {
400    checkNotNull(iterable);
401    return new Iterator<T>() {
402      Iterator<T> iterator = emptyModifiableIterator();
403
404      @Override
405      public boolean hasNext() {
406        /*
407         * Don't store a new Iterator until we know the user can't remove() the last returned
408         * element anymore. Otherwise, when we remove from the old iterator, we may be invalidating
409         * the new one. The result is a ConcurrentModificationException or other bad behavior.
410         *
411         * (If we decide that we really, really hate allocating two Iterators per cycle instead of
412         * one, we can optimistically store the new Iterator and then be willing to throw it out if
413         * the user calls remove().)
414         */
415        return iterator.hasNext() || iterable.iterator().hasNext();
416      }
417
418      @Override
419      @ParametricNullness
420      public T next() {
421        if (!iterator.hasNext()) {
422          iterator = iterable.iterator();
423          if (!iterator.hasNext()) {
424            throw new NoSuchElementException();
425          }
426        }
427        return iterator.next();
428      }
429
430      @Override
431      public void remove() {
432        iterator.remove();
433      }
434    };
435  }
436
437  /**
438   * Returns an iterator that cycles indefinitely over the provided elements.
439   *
440   * <p>The returned iterator supports {@code remove()}. After {@code remove()} is called,
441   * subsequent cycles omit the removed element, but {@code elements} does not change. The
442   * iterator's {@code hasNext()} method returns {@code true} until all of the original elements
443   * have been removed.
444   *
445   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
446   * should use an explicit {@code break} or be certain that you will eventually remove all the
447   * elements.
448   */
449  @SafeVarargs
450  public static <T extends @Nullable Object> Iterator<T> cycle(T... elements) {
451    return cycle(Lists.newArrayList(elements));
452  }
453
454  /**
455   * Returns an Iterator that walks the specified array, nulling out elements behind it. This can
456   * avoid memory leaks when an element is no longer necessary.
457   *
458   * <p>This method accepts an array with element type {@code @Nullable T}, but callers must pass an
459   * array whose contents are initially non-null. The {@code @Nullable} annotation indicates that
460   * this method will write nulls into the array during iteration.
461   *
462   * <p>This is mainly just to avoid the intermediate ArrayDeque in ConsumingQueueIterator.
463   */
464  private static <I extends Iterator<?>> Iterator<I> consumingForArray(@Nullable I... elements) {
465    return new UnmodifiableIterator<I>() {
466      int index = 0;
467
468      @Override
469      public boolean hasNext() {
470        return index < elements.length;
471      }
472
473      @Override
474      public I next() {
475        if (!hasNext()) {
476          throw new NoSuchElementException();
477        }
478        /*
479         * requireNonNull is safe because our callers always pass non-null arguments. Each element
480         * of the array becomes null only when we iterate past it and then clear it.
481         */
482        I result = requireNonNull(elements[index]);
483        elements[index] = null;
484        index++;
485        return result;
486      }
487    };
488  }
489
490  /**
491   * Combines two iterators into a single iterator. The returned iterator iterates across the
492   * elements in {@code a}, followed by the elements in {@code b}. The source iterators are not
493   * polled until necessary.
494   *
495   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
496   * supports it.
497   */
498  public static <T extends @Nullable Object> Iterator<T> concat(
499      Iterator<? extends T> a, Iterator<? extends T> b) {
500    checkNotNull(a);
501    checkNotNull(b);
502    return concat(consumingForArray(a, b));
503  }
504
505  /**
506   * Combines three iterators into a single iterator. The returned iterator iterates across the
507   * elements in {@code a}, followed by the elements in {@code b}, followed by the elements in
508   * {@code c}. The source iterators are not polled until necessary.
509   *
510   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
511   * supports it.
512   */
513  public static <T extends @Nullable Object> Iterator<T> concat(
514      Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c) {
515    checkNotNull(a);
516    checkNotNull(b);
517    checkNotNull(c);
518    return concat(consumingForArray(a, b, c));
519  }
520
521  /**
522   * Combines four iterators into a single iterator. The returned iterator iterates across the
523   * elements in {@code a}, followed by the elements in {@code b}, followed by the elements in
524   * {@code c}, followed by the elements in {@code d}. The source iterators are not polled until
525   * necessary.
526   *
527   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
528   * supports it.
529   */
530  public static <T extends @Nullable Object> Iterator<T> concat(
531      Iterator<? extends T> a,
532      Iterator<? extends T> b,
533      Iterator<? extends T> c,
534      Iterator<? extends T> d) {
535    checkNotNull(a);
536    checkNotNull(b);
537    checkNotNull(c);
538    checkNotNull(d);
539    return concat(consumingForArray(a, b, c, d));
540  }
541
542  /**
543   * Combines multiple iterators into a single iterator. The returned iterator iterates across the
544   * elements of each iterator in {@code inputs}. The input iterators are not polled until
545   * necessary.
546   *
547   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
548   * supports it.
549   *
550   * @throws NullPointerException if any of the provided iterators is null
551   */
552  @SafeVarargs
553  public static <T extends @Nullable Object> Iterator<T> concat(Iterator<? extends T>... inputs) {
554    return concatNoDefensiveCopy(Arrays.copyOf(inputs, inputs.length));
555  }
556
557  /**
558   * Combines multiple iterators into a single iterator. The returned iterator iterates across the
559   * elements of each iterator in {@code inputs}. The input iterators are not polled until
560   * necessary.
561   *
562   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
563   * supports it. The methods of the returned iterator may throw {@code NullPointerException} if any
564   * of the input iterators is null.
565   */
566  public static <T extends @Nullable Object> Iterator<T> concat(
567      Iterator<? extends Iterator<? extends T>> inputs) {
568    return new ConcatenatedIterator<>(inputs);
569  }
570
571  /** Concats a varargs array of iterators without making a defensive copy of the array. */
572  static <T extends @Nullable Object> Iterator<T> concatNoDefensiveCopy(
573      Iterator<? extends T>... inputs) {
574    for (Iterator<? extends T> input : checkNotNull(inputs)) {
575      checkNotNull(input);
576    }
577    return concat(consumingForArray(inputs));
578  }
579
580  /**
581   * Divides an iterator into unmodifiable sublists of the given size (the final list may be
582   * smaller). For example, partitioning an iterator containing {@code [a, b, c, d, e]} with a
583   * partition size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer iterator containing two
584   * inner lists of three and two elements, all in the original order.
585   *
586   * <p>The returned lists implement {@link java.util.RandomAccess}.
587   *
588   * <p><b>Note:</b> The current implementation eagerly allocates storage for {@code size} elements.
589   * As a consequence, passing values like {@code Integer.MAX_VALUE} can lead to {@link
590   * OutOfMemoryError}.
591   *
592   * @param iterator the iterator to return a partitioned view of
593   * @param size the desired size of each partition (the last may be smaller)
594   * @return an iterator of immutable lists containing the elements of {@code iterator} divided into
595   *     partitions
596   * @throws IllegalArgumentException if {@code size} is nonpositive
597   */
598  public static <T extends @Nullable Object> UnmodifiableIterator<List<T>> partition(
599      Iterator<T> iterator, int size) {
600    return partitionImpl(iterator, size, false);
601  }
602
603  /**
604   * Divides an iterator into unmodifiable sublists of the given size, padding the final iterator
605   * with null values if necessary. For example, partitioning an iterator containing {@code [a, b,
606   * c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]} -- an outer
607   * iterator containing two inner lists of three elements each, all in the original order.
608   *
609   * <p>The returned lists implement {@link java.util.RandomAccess}.
610   *
611   * @param iterator the iterator to return a partitioned view of
612   * @param size the desired size of each partition
613   * @return an iterator of immutable lists containing the elements of {@code iterator} divided into
614   *     partitions (the final iterable may have trailing null elements)
615   * @throws IllegalArgumentException if {@code size} is nonpositive
616   */
617  public static <T extends @Nullable Object>
618      UnmodifiableIterator<List<@Nullable T>> paddedPartition(Iterator<T> iterator, int size) {
619    return partitionImpl(iterator, size, true);
620  }
621
622  private static <T extends @Nullable Object> UnmodifiableIterator<List<@Nullable T>> partitionImpl(
623      Iterator<T> iterator, int size, boolean pad) {
624    checkNotNull(iterator);
625    checkArgument(size > 0);
626    return new UnmodifiableIterator<List<@Nullable T>>() {
627      @Override
628      public boolean hasNext() {
629        return iterator.hasNext();
630      }
631
632      @Override
633      public List<@Nullable T> next() {
634        if (!hasNext()) {
635          throw new NoSuchElementException();
636        }
637        @SuppressWarnings("unchecked") // we only put Ts in it
638        @Nullable
639        T[] array = (@Nullable T[]) new Object[size];
640        int count = 0;
641        for (; count < size && iterator.hasNext(); count++) {
642          array[count] = iterator.next();
643        }
644        for (int i = count; i < size; i++) {
645          array[i] = null; // for GWT
646        }
647
648        List<@Nullable T> list = Collections.unmodifiableList(Arrays.asList(array));
649        // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
650        if (pad || count == size) {
651          return list;
652        } else {
653          return list.subList(0, count);
654        }
655      }
656    };
657  }
658
659  /**
660   * Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
661   * {@code retainIfTrue}.
662   */
663  public static <T extends @Nullable Object> UnmodifiableIterator<T> filter(
664      Iterator<T> unfiltered, Predicate<? super T> retainIfTrue) {
665    checkNotNull(unfiltered);
666    checkNotNull(retainIfTrue);
667    return new AbstractIterator<T>() {
668      @Override
669      @CheckForNull
670      protected T computeNext() {
671        while (unfiltered.hasNext()) {
672          T element = unfiltered.next();
673          if (retainIfTrue.apply(element)) {
674            return element;
675          }
676        }
677        return endOfData();
678      }
679    };
680  }
681
682  /**
683   * Returns a view of {@code unfiltered} containing all elements that are of the type {@code
684   * desiredType}.
685   */
686  @SuppressWarnings("unchecked") // can cast to <T> because non-Ts are removed
687  @GwtIncompatible // Class.isInstance
688  public static <T> UnmodifiableIterator<T> filter(Iterator<?> unfiltered, Class<T> desiredType) {
689    return (UnmodifiableIterator<T>) filter(unfiltered, instanceOf(desiredType));
690  }
691
692  /**
693   * Returns {@code true} if one or more elements returned by {@code iterator} satisfy the given
694   * predicate.
695   */
696  public static <T extends @Nullable Object> boolean any(
697      Iterator<T> iterator, Predicate<? super T> predicate) {
698    return indexOf(iterator, predicate) != -1;
699  }
700
701  /**
702   * Returns {@code true} if every element returned by {@code iterator} satisfies the given
703   * predicate. If {@code iterator} is empty, {@code true} is returned.
704   */
705  public static <T extends @Nullable Object> boolean all(
706      Iterator<T> iterator, Predicate<? super T> predicate) {
707    checkNotNull(predicate);
708    while (iterator.hasNext()) {
709      T element = iterator.next();
710      if (!predicate.apply(element)) {
711        return false;
712      }
713    }
714    return true;
715  }
716
717  /**
718   * Returns the first element in {@code iterator} that satisfies the given predicate; use this
719   * method only when such an element is known to exist. If no such element is found, the iterator
720   * will be left exhausted: its {@code hasNext()} method will return {@code false}. If it is
721   * possible that <i>no</i> element will match, use {@link #tryFind} or {@link #find(Iterator,
722   * Predicate, Object)} instead.
723   *
724   * @throws NoSuchElementException if no element in {@code iterator} matches the given predicate
725   */
726  @ParametricNullness
727  public static <T extends @Nullable Object> T find(
728      Iterator<T> iterator, Predicate<? super T> predicate) {
729    checkNotNull(iterator);
730    checkNotNull(predicate);
731    while (iterator.hasNext()) {
732      T t = iterator.next();
733      if (predicate.apply(t)) {
734        return t;
735      }
736    }
737    throw new NoSuchElementException();
738  }
739
740  /**
741   * Returns the first element in {@code iterator} that satisfies the given predicate. If no such
742   * element is found, {@code defaultValue} will be returned from this method and the iterator will
743   * be left exhausted: its {@code hasNext()} method will return {@code false}. Note that this can
744   * usually be handled more naturally using {@code tryFind(iterator, predicate).or(defaultValue)}.
745   *
746   * @since 7.0
747   */
748  // For discussion of this signature, see the corresponding overload of *Iterables*.find.
749  @CheckForNull
750  public static <T extends @Nullable Object> T find(
751      Iterator<? extends T> iterator,
752      Predicate<? super T> predicate,
753      @CheckForNull T defaultValue) {
754    checkNotNull(iterator);
755    checkNotNull(predicate);
756    while (iterator.hasNext()) {
757      T t = iterator.next();
758      if (predicate.apply(t)) {
759        return t;
760      }
761    }
762    return defaultValue;
763  }
764
765  /**
766   * Returns an {@link Optional} containing the first element in {@code iterator} that satisfies the
767   * given predicate, if such an element exists. If no such element is found, an empty {@link
768   * Optional} will be returned from this method and the iterator will be left exhausted: its {@code
769   * hasNext()} method will return {@code false}.
770   *
771   * <p><b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If {@code null}
772   * is matched in {@code iterator}, a NullPointerException will be thrown.
773   *
774   * @since 11.0
775   */
776  public static <T> Optional<T> tryFind(Iterator<T> iterator, Predicate<? super T> predicate) {
777    checkNotNull(iterator);
778    checkNotNull(predicate);
779    while (iterator.hasNext()) {
780      T t = iterator.next();
781      if (predicate.apply(t)) {
782        return Optional.of(t);
783      }
784    }
785    return Optional.absent();
786  }
787
788  /**
789   * Returns the index in {@code iterator} of the first element that satisfies the provided {@code
790   * predicate}, or {@code -1} if the Iterator has no such elements.
791   *
792   * <p>More formally, returns the lowest index {@code i} such that {@code
793   * predicate.apply(Iterators.get(iterator, i))} returns {@code true}, or {@code -1} if there is no
794   * such index.
795   *
796   * <p>If -1 is returned, the iterator will be left exhausted: its {@code hasNext()} method will
797   * return {@code false}. Otherwise, the iterator will be set to the element which satisfies the
798   * {@code predicate}.
799   *
800   * @since 2.0
801   */
802  public static <T extends @Nullable Object> int indexOf(
803      Iterator<T> iterator, Predicate<? super T> predicate) {
804    checkNotNull(predicate, "predicate");
805    for (int i = 0; iterator.hasNext(); i++) {
806      T current = iterator.next();
807      if (predicate.apply(current)) {
808        return i;
809      }
810    }
811    return -1;
812  }
813
814  /**
815   * Returns a view containing the result of applying {@code function} to each element of {@code
816   * fromIterator}.
817   *
818   * <p>The returned iterator supports {@code remove()} if {@code fromIterator} does. After a
819   * successful {@code remove()} call, {@code fromIterator} no longer contains the corresponding
820   * element.
821   */
822  public static <F extends @Nullable Object, T extends @Nullable Object> Iterator<T> transform(
823      Iterator<F> fromIterator, Function<? super F, ? extends T> function) {
824    checkNotNull(function);
825    return new TransformedIterator<F, T>(fromIterator) {
826      @ParametricNullness
827      @Override
828      T transform(@ParametricNullness F from) {
829        return function.apply(from);
830      }
831    };
832  }
833
834  /**
835   * Advances {@code iterator} {@code position + 1} times, returning the element at the {@code
836   * position}th position.
837   *
838   * @param position position of the element to return
839   * @return the element at the specified position in {@code iterator}
840   * @throws IndexOutOfBoundsException if {@code position} is negative or greater than or equal to
841   *     the number of elements remaining in {@code iterator}
842   */
843  @ParametricNullness
844  public static <T extends @Nullable Object> T get(Iterator<T> iterator, int position) {
845    checkNonnegative(position);
846    int skipped = advance(iterator, position);
847    if (!iterator.hasNext()) {
848      throw new IndexOutOfBoundsException(
849          "position ("
850              + position
851              + ") must be less than the number of elements that remained ("
852              + skipped
853              + ")");
854    }
855    return iterator.next();
856  }
857
858  /**
859   * Advances {@code iterator} {@code position + 1} times, returning the element at the {@code
860   * position}th position or {@code defaultValue} otherwise.
861   *
862   * @param position position of the element to return
863   * @param defaultValue the default value to return if the iterator is empty or if {@code position}
864   *     is greater than the number of elements remaining in {@code iterator}
865   * @return the element at the specified position in {@code iterator} or {@code defaultValue} if
866   *     {@code iterator} produces fewer than {@code position + 1} elements.
867   * @throws IndexOutOfBoundsException if {@code position} is negative
868   * @since 4.0
869   */
870  @ParametricNullness
871  public static <T extends @Nullable Object> T get(
872      Iterator<? extends T> iterator, int position, @ParametricNullness T defaultValue) {
873    checkNonnegative(position);
874    advance(iterator, position);
875    return getNext(iterator, defaultValue);
876  }
877
878  static void checkNonnegative(int position) {
879    if (position < 0) {
880      throw new IndexOutOfBoundsException("position (" + position + ") must not be negative");
881    }
882  }
883
884  /**
885   * Returns the next element in {@code iterator} or {@code defaultValue} if the iterator is empty.
886   * The {@link Iterables} analog to this method is {@link Iterables#getFirst}.
887   *
888   * @param defaultValue the default value to return if the iterator is empty
889   * @return the next element of {@code iterator} or the default value
890   * @since 7.0
891   */
892  @ParametricNullness
893  public static <T extends @Nullable Object> T getNext(
894      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
895    return iterator.hasNext() ? iterator.next() : defaultValue;
896  }
897
898  /**
899   * Advances {@code iterator} to the end, returning the last element.
900   *
901   * @return the last element of {@code iterator}
902   * @throws NoSuchElementException if the iterator is empty
903   */
904  @ParametricNullness
905  public static <T extends @Nullable Object> T getLast(Iterator<T> iterator) {
906    while (true) {
907      T current = iterator.next();
908      if (!iterator.hasNext()) {
909        return current;
910      }
911    }
912  }
913
914  /**
915   * Advances {@code iterator} to the end, returning the last element or {@code defaultValue} if the
916   * iterator is empty.
917   *
918   * @param defaultValue the default value to return if the iterator is empty
919   * @return the last element of {@code iterator}
920   * @since 3.0
921   */
922  @ParametricNullness
923  public static <T extends @Nullable Object> T getLast(
924      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
925    return iterator.hasNext() ? getLast(iterator) : defaultValue;
926  }
927
928  /**
929   * Calls {@code next()} on {@code iterator}, either {@code numberToAdvance} times or until {@code
930   * hasNext()} returns {@code false}, whichever comes first.
931   *
932   * @return the number of elements the iterator was advanced
933   * @since 13.0 (since 3.0 as {@code Iterators.skip})
934   */
935  @CanIgnoreReturnValue
936  public static int advance(Iterator<?> iterator, int numberToAdvance) {
937    checkNotNull(iterator);
938    checkArgument(numberToAdvance >= 0, "numberToAdvance must be nonnegative");
939
940    int i;
941    for (i = 0; i < numberToAdvance && iterator.hasNext(); i++) {
942      iterator.next();
943    }
944    return i;
945  }
946
947  /**
948   * Returns a view containing the first {@code limitSize} elements of {@code iterator}. If {@code
949   * iterator} contains fewer than {@code limitSize} elements, the returned view contains all of its
950   * elements. The returned iterator supports {@code remove()} if {@code iterator} does.
951   *
952   * @param iterator the iterator to limit
953   * @param limitSize the maximum number of elements in the returned iterator
954   * @throws IllegalArgumentException if {@code limitSize} is negative
955   * @since 3.0
956   */
957  public static <T extends @Nullable Object> Iterator<T> limit(
958      Iterator<T> iterator, int limitSize) {
959    checkNotNull(iterator);
960    checkArgument(limitSize >= 0, "limit is negative");
961    return new Iterator<T>() {
962      private int count;
963
964      @Override
965      public boolean hasNext() {
966        return count < limitSize && iterator.hasNext();
967      }
968
969      @Override
970      @ParametricNullness
971      public T next() {
972        if (!hasNext()) {
973          throw new NoSuchElementException();
974        }
975        count++;
976        return iterator.next();
977      }
978
979      @Override
980      public void remove() {
981        iterator.remove();
982      }
983    };
984  }
985
986  /**
987   * Returns a view of the supplied {@code iterator} that removes each element from the supplied
988   * {@code iterator} as it is returned.
989   *
990   * <p>The provided iterator must support {@link Iterator#remove()} or else the returned iterator
991   * will fail on the first call to {@code next}. The returned {@link Iterator} is also not
992   * thread-safe.
993   *
994   * @param iterator the iterator to remove and return elements from
995   * @return an iterator that removes and returns elements from the supplied iterator
996   * @since 2.0
997   */
998  public static <T extends @Nullable Object> Iterator<T> consumingIterator(Iterator<T> iterator) {
999    checkNotNull(iterator);
1000    return new UnmodifiableIterator<T>() {
1001      @Override
1002      public boolean hasNext() {
1003        return iterator.hasNext();
1004      }
1005
1006      @Override
1007      @ParametricNullness
1008      public T next() {
1009        T next = iterator.next();
1010        iterator.remove();
1011        return next;
1012      }
1013
1014      @Override
1015      public String toString() {
1016        return "Iterators.consumingIterator(...)";
1017      }
1018    };
1019  }
1020
1021  /**
1022   * Deletes and returns the next value from the iterator, or returns {@code null} if there is no
1023   * such value.
1024   */
1025  @CheckForNull
1026  static <T extends @Nullable Object> T pollNext(Iterator<T> iterator) {
1027    if (iterator.hasNext()) {
1028      T result = iterator.next();
1029      iterator.remove();
1030      return result;
1031    } else {
1032      return null;
1033    }
1034  }
1035
1036  // Methods only in Iterators, not in Iterables
1037
1038  /** Clears the iterator using its remove method. */
1039  static void clear(Iterator<?> iterator) {
1040    checkNotNull(iterator);
1041    while (iterator.hasNext()) {
1042      iterator.next();
1043      iterator.remove();
1044    }
1045  }
1046
1047  /**
1048   * Returns an iterator containing the elements of {@code array} in order. The returned iterator is
1049   * a view of the array; subsequent changes to the array will be reflected in the iterator.
1050   *
1051   * <p><b>Note:</b> It is often preferable to represent your data using a collection type, for
1052   * example using {@link Arrays#asList(Object[])}, making this method unnecessary.
1053   *
1054   * <p>The {@code Iterable} equivalent of this method is either {@link Arrays#asList(Object[])},
1055   * {@link ImmutableList#copyOf(Object[])}}, or {@link ImmutableList#of}.
1056   */
1057  @SafeVarargs
1058  public static <T extends @Nullable Object> UnmodifiableIterator<T> forArray(T... array) {
1059    return forArrayWithPosition(array, 0);
1060  }
1061
1062  /**
1063   * Returns a list iterator containing the elements in the specified {@code array} in order,
1064   * starting at the specified {@code position}.
1065   *
1066   * <p>The {@code Iterable} equivalent of this method is {@code
1067   * Arrays.asList(array).listIterator(position)}.
1068   */
1069  static <T extends @Nullable Object> UnmodifiableListIterator<T> forArrayWithPosition(
1070      T[] array, int position) {
1071    if (array.length == 0) {
1072      Preconditions.checkPositionIndex(position, array.length); // otherwise checked in ArrayItr
1073      return emptyListIterator();
1074    }
1075    return new ArrayItr<>(array, position);
1076  }
1077
1078  private static final class ArrayItr<T extends @Nullable Object>
1079      extends AbstractIndexedListIterator<T> {
1080    static final UnmodifiableListIterator<Object> EMPTY = new ArrayItr<>(new Object[0], 0);
1081
1082    private final T[] array;
1083
1084    ArrayItr(T[] array, int position) {
1085      super(array.length, position);
1086      this.array = array;
1087    }
1088
1089    @Override
1090    @ParametricNullness
1091    protected T get(int index) {
1092      return array[index];
1093    }
1094  }
1095
1096  /**
1097   * Returns an iterator containing only {@code value}.
1098   *
1099   * <p>The {@link Iterable} equivalent of this method is {@link Collections#singleton}.
1100   */
1101  public static <T extends @Nullable Object> UnmodifiableIterator<T> singletonIterator(
1102      @ParametricNullness T value) {
1103    return new SingletonIterator<>(value);
1104  }
1105
1106  private static final class SingletonIterator<T extends @Nullable Object>
1107      extends UnmodifiableIterator<T> {
1108    private static final Object SENTINEL = new Object();
1109
1110    private @Nullable Object valueOrSentinel;
1111
1112    SingletonIterator(T value) {
1113      this.valueOrSentinel = value;
1114    }
1115
1116    @Override
1117    public boolean hasNext() {
1118      return valueOrSentinel != SENTINEL;
1119    }
1120
1121    @Override
1122    @ParametricNullness
1123    public T next() {
1124      if (valueOrSentinel == SENTINEL) {
1125        throw new NoSuchElementException();
1126      }
1127      // The field held either a T or SENTINEL, and it turned out not to be SENTINEL.
1128      @SuppressWarnings("unchecked")
1129      T t = (T) valueOrSentinel;
1130      valueOrSentinel = SENTINEL;
1131      return t;
1132    }
1133  }
1134
1135  /**
1136   * Adapts an {@code Enumeration} to the {@code Iterator} interface.
1137   *
1138   * <p>This method has no equivalent in {@link Iterables} because viewing an {@code Enumeration} as
1139   * an {@code Iterable} is impossible. However, the contents can be <i>copied</i> into a collection
1140   * using {@link Collections#list}.
1141   *
1142   * <p><b>Java 9 users:</b> use {@code enumeration.asIterator()} instead, unless it is important to
1143   * return an {@code UnmodifiableIterator} instead of a plain {@code Iterator}.
1144   */
1145  public static <T extends @Nullable Object> UnmodifiableIterator<T> forEnumeration(
1146      Enumeration<T> enumeration) {
1147    checkNotNull(enumeration);
1148    return new UnmodifiableIterator<T>() {
1149      @Override
1150      public boolean hasNext() {
1151        return enumeration.hasMoreElements();
1152      }
1153
1154      @Override
1155      @ParametricNullness
1156      public T next() {
1157        return enumeration.nextElement();
1158      }
1159    };
1160  }
1161
1162  /**
1163   * Adapts an {@code Iterator} to the {@code Enumeration} interface.
1164   *
1165   * <p>The {@code Iterable} equivalent of this method is either {@link Collections#enumeration} (if
1166   * you have a {@link Collection}), or {@code Iterators.asEnumeration(collection.iterator())}.
1167   */
1168  public static <T extends @Nullable Object> Enumeration<T> asEnumeration(Iterator<T> iterator) {
1169    checkNotNull(iterator);
1170    return new Enumeration<T>() {
1171      @Override
1172      public boolean hasMoreElements() {
1173        return iterator.hasNext();
1174      }
1175
1176      @Override
1177      @ParametricNullness
1178      public T nextElement() {
1179        return iterator.next();
1180      }
1181    };
1182  }
1183
1184  /** Implementation of PeekingIterator that avoids peeking unless necessary. */
1185  private static class PeekingImpl<E extends @Nullable Object> implements PeekingIterator<E> {
1186
1187    private final Iterator<? extends E> iterator;
1188    private boolean hasPeeked;
1189    @CheckForNull private E peekedElement;
1190
1191    public PeekingImpl(Iterator<? extends E> iterator) {
1192      this.iterator = checkNotNull(iterator);
1193    }
1194
1195    @Override
1196    public boolean hasNext() {
1197      return hasPeeked || iterator.hasNext();
1198    }
1199
1200    @Override
1201    @ParametricNullness
1202    public E next() {
1203      if (!hasPeeked) {
1204        return iterator.next();
1205      }
1206      // The cast is safe because of the hasPeeked check.
1207      E result = uncheckedCastNullableTToT(peekedElement);
1208      hasPeeked = false;
1209      peekedElement = null;
1210      return result;
1211    }
1212
1213    @Override
1214    public void remove() {
1215      checkState(!hasPeeked, "Can't remove after you've peeked at next");
1216      iterator.remove();
1217    }
1218
1219    @Override
1220    @ParametricNullness
1221    public E peek() {
1222      if (!hasPeeked) {
1223        peekedElement = iterator.next();
1224        hasPeeked = true;
1225      }
1226      // The cast is safe because of the hasPeeked check.
1227      return uncheckedCastNullableTToT(peekedElement);
1228    }
1229  }
1230
1231  /**
1232   * Returns a {@code PeekingIterator} backed by the given iterator.
1233   *
1234   * <p>Calls to the {@code peek} method with no intervening calls to {@code next} do not affect the
1235   * iteration, and hence return the same object each time. A subsequent call to {@code next} is
1236   * guaranteed to return the same object again. For example:
1237   *
1238   * <pre>{@code
1239   * PeekingIterator<String> peekingIterator =
1240   *     Iterators.peekingIterator(Iterators.forArray("a", "b"));
1241   * String a1 = peekingIterator.peek(); // returns "a"
1242   * String a2 = peekingIterator.peek(); // also returns "a"
1243   * String a3 = peekingIterator.next(); // also returns "a"
1244   * }</pre>
1245   *
1246   * <p>Any structural changes to the underlying iteration (aside from those performed by the
1247   * iterator's own {@link PeekingIterator#remove()} method) will leave the iterator in an undefined
1248   * state.
1249   *
1250   * <p>The returned iterator does not support removal after peeking, as explained by {@link
1251   * PeekingIterator#remove()}.
1252   *
1253   * <p>Note: If the given iterator is already a {@code PeekingIterator}, it <i>might</i> be
1254   * returned to the caller, although this is neither guaranteed to occur nor required to be
1255   * consistent. For example, this method <i>might</i> choose to pass through recognized
1256   * implementations of {@code PeekingIterator} when the behavior of the implementation is known to
1257   * meet the contract guaranteed by this method.
1258   *
1259   * <p>There is no {@link Iterable} equivalent to this method, so use this method to wrap each
1260   * individual iterator as it is generated.
1261   *
1262   * @param iterator the backing iterator. The {@link PeekingIterator} assumes ownership of this
1263   *     iterator, so users should cease making direct calls to it after calling this method.
1264   * @return a peeking iterator backed by that iterator. Apart from the additional {@link
1265   *     PeekingIterator#peek()} method, this iterator behaves exactly the same as {@code iterator}.
1266   */
1267  public static <T extends @Nullable Object> PeekingIterator<T> peekingIterator(
1268      Iterator<? extends T> iterator) {
1269    if (iterator instanceof PeekingImpl) {
1270      // Safe to cast <? extends T> to <T> because PeekingImpl only uses T
1271      // covariantly (and cannot be subclassed to add non-covariant uses).
1272      @SuppressWarnings("unchecked")
1273      PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
1274      return peeking;
1275    }
1276    return new PeekingImpl<>(iterator);
1277  }
1278
1279  /**
1280   * Simply returns its argument.
1281   *
1282   * @deprecated no need to use this
1283   * @since 10.0
1284   */
1285  @Deprecated
1286  public static <T extends @Nullable Object> PeekingIterator<T> peekingIterator(
1287      PeekingIterator<T> iterator) {
1288    return checkNotNull(iterator);
1289  }
1290
1291  /**
1292   * Returns an iterator over the merged contents of all given {@code iterators}, traversing every
1293   * element of the input iterators. Equivalent entries will not be de-duplicated.
1294   *
1295   * <p>Callers must ensure that the source {@code iterators} are in non-descending order as this
1296   * method does not sort its input.
1297   *
1298   * <p>For any equivalent elements across all {@code iterators}, it is undefined which element is
1299   * returned first.
1300   *
1301   * @since 11.0
1302   */
1303  public static <T extends @Nullable Object> UnmodifiableIterator<T> mergeSorted(
1304      Iterable<? extends Iterator<? extends T>> iterators, Comparator<? super T> comparator) {
1305    checkNotNull(iterators, "iterators");
1306    checkNotNull(comparator, "comparator");
1307
1308    return new MergingIterator<>(iterators, comparator);
1309  }
1310
1311  /**
1312   * An iterator that performs a lazy N-way merge, calculating the next value each time the iterator
1313   * is polled. This amortizes the sorting cost over the iteration and requires less memory than
1314   * sorting all elements at once.
1315   *
1316   * <p>Retrieving a single element takes approximately O(log(M)) time, where M is the number of
1317   * iterators. (Retrieving all elements takes approximately O(N*log(M)) time, where N is the total
1318   * number of elements.)
1319   */
1320  private static class MergingIterator<T extends @Nullable Object> extends UnmodifiableIterator<T> {
1321    final Queue<PeekingIterator<T>> queue;
1322
1323    public MergingIterator(
1324        Iterable<? extends Iterator<? extends T>> iterators, Comparator<? super T> itemComparator) {
1325      // A comparator that's used by the heap, allowing the heap
1326      // to be sorted based on the top of each iterator.
1327      Comparator<PeekingIterator<T>> heapComparator =
1328          (PeekingIterator<T> o1, PeekingIterator<T> o2) ->
1329              itemComparator.compare(o1.peek(), o2.peek());
1330
1331      queue = new PriorityQueue<>(2, heapComparator);
1332
1333      for (Iterator<? extends T> iterator : iterators) {
1334        if (iterator.hasNext()) {
1335          queue.add(Iterators.peekingIterator(iterator));
1336        }
1337      }
1338    }
1339
1340    @Override
1341    public boolean hasNext() {
1342      return !queue.isEmpty();
1343    }
1344
1345    @Override
1346    @ParametricNullness
1347    public T next() {
1348      PeekingIterator<T> nextIter = queue.remove();
1349      T next = nextIter.next();
1350      if (nextIter.hasNext()) {
1351        queue.add(nextIter);
1352      }
1353      return next;
1354    }
1355  }
1356
1357  private static class ConcatenatedIterator<T extends @Nullable Object> implements Iterator<T> {
1358    /* The last iterator to return an element.  Calls to remove() go to this iterator. */
1359    @CheckForNull private Iterator<? extends T> toRemove;
1360
1361    /* The iterator currently returning elements. */
1362    private Iterator<? extends T> iterator;
1363
1364    /*
1365     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
1366     * is the only one in use, but if we encounter nested concatenations, we start a deque of
1367     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
1368     * operation O(1).
1369     */
1370
1371    @CheckForNull private Iterator<? extends Iterator<? extends T>> topMetaIterator;
1372
1373    // Only becomes nonnull if we encounter nested concatenations.
1374    @CheckForNull private Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;
1375
1376    ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
1377      iterator = emptyIterator();
1378      topMetaIterator = checkNotNull(metaIterator);
1379    }
1380
1381    // Returns a nonempty meta-iterator or, if all meta-iterators are empty, null.
1382    @CheckForNull
1383    private Iterator<? extends Iterator<? extends T>> getTopMetaIterator() {
1384      while (topMetaIterator == null || !topMetaIterator.hasNext()) {
1385        if (metaIterators != null && !metaIterators.isEmpty()) {
1386          topMetaIterator = metaIterators.removeFirst();
1387        } else {
1388          return null;
1389        }
1390      }
1391      return topMetaIterator;
1392    }
1393
1394    @Override
1395    public boolean hasNext() {
1396      while (!checkNotNull(iterator).hasNext()) {
1397        // this weird checkNotNull positioning appears required by our tests, which expect
1398        // both hasNext and next to throw NPE if an input iterator is null.
1399
1400        topMetaIterator = getTopMetaIterator();
1401        if (topMetaIterator == null) {
1402          return false;
1403        }
1404
1405        iterator = topMetaIterator.next();
1406
1407        if (iterator instanceof ConcatenatedIterator) {
1408          // Instead of taking linear time in the number of nested concatenations, unpack
1409          // them into the queue
1410          @SuppressWarnings("unchecked")
1411          ConcatenatedIterator<T> topConcat = (ConcatenatedIterator<T>) iterator;
1412          iterator = topConcat.iterator;
1413
1414          // topConcat.topMetaIterator, then topConcat.metaIterators, then this.topMetaIterator,
1415          // then this.metaIterators
1416
1417          if (this.metaIterators == null) {
1418            this.metaIterators = new ArrayDeque<>();
1419          }
1420          this.metaIterators.addFirst(this.topMetaIterator);
1421          if (topConcat.metaIterators != null) {
1422            while (!topConcat.metaIterators.isEmpty()) {
1423              this.metaIterators.addFirst(topConcat.metaIterators.removeLast());
1424            }
1425          }
1426          this.topMetaIterator = topConcat.topMetaIterator;
1427        }
1428      }
1429      return true;
1430    }
1431
1432    @Override
1433    @ParametricNullness
1434    public T next() {
1435      if (hasNext()) {
1436        toRemove = iterator;
1437        return iterator.next();
1438      } else {
1439        throw new NoSuchElementException();
1440      }
1441    }
1442
1443    @Override
1444    public void remove() {
1445      if (toRemove == null) {
1446        throw new IllegalStateException("no calls to next() since the last call to remove()");
1447      }
1448      toRemove.remove();
1449      toRemove = null;
1450    }
1451  }
1452}